1. Field of the Invention
The present invention relates to an optical integrated circuit element usable for optical communications, optical information processing, optical sensing, and the like. More specifically, the present invention relates to a waveguide type optical integrated circuit element where a semiconductor laser which acts as a light emitting device and an optical waveguide for propagating light output from the semiconductor laser are integrally formed on a same semiconductor substrate, and a method for fabricating such a waveguide type optical integrated circuit element.
2. Description of the Related Art
With the present rapid progress in multimedia society, it is anticipated that optical communications with a large capacity and a high speed of 100 Mbps or more will become available at home in near future. In particular, the development of wireless optical transmission technology not only makes wirings for communications unnecessary, but also provides a great benefit in realizing a communication link using a portable computer via a terminal at a nearby relay point.
FIG. 6 shows an example of a conventional waveguide type optical integrated circuit element used as a receiver section of a wireless optical communication system.
The wireless optical communication system adopts a heterodyne wave detection method where frequency-modulated signal light 612 is combined with locally oscillated light 611 in the receiver section, to be converted into a beat signal having a frequency identical to the difference frequency. This method is advantageous over a general intensity modulation direct detection method in that the communication is excellent because a good signal to noise characteristic can be realized.
Referring to FIG. 6, the configuration of the conventional waveguide type optical integrated circuit element will be described together with the operation thereof. The waveguide type optical integrated circuit element includes a semiconductor laser 200 and two combinations of optical waveguides 630, 631 and 632, 633, which are integrally formed on a same substrate 100. An optical branching element 620 is also integrally formed at the crossing of the two combinations of optical waveguides 630, 631 and 632, 633.
The locally oscillated light 611 emitted from the semiconductor laser 200 is introduced into the input-side optical waveguide 630 among the integrally-formed optical waveguides. The light is then branched into two by the optical branching element 620 to be introduced into the output-side optical waveguides 631 and 633.
On the other hand, the transmitted signal light 612 is introduced into the input-side optical waveguide 632. The light is then branched into two by the optical branching element 620 to be introduced into the output-side optical waveguides 631 and 633. As a result, the locally oscillated light 611 and the signal light 612 are combined in the output-side optical waveguides 631 and 633, so as to obtain beat signals.
In the fabrication of the waveguide type optical integrated circuit element with the above configuration, it is required to form the semiconductor laser and the optical waveguide integrally on a same substrate. One example of the method for realizing this integration is an abutting method as shown in FIG. 7A. Referring to FIG. 7A, which shows an ideal integration by the abutting method, a distributed feedback (DFB) type semiconductor laser 200 formed on a semiconductor substrate 100 is vertically etched to remove part thereof, and an optical waveguide structure 300 is formed in the etched area. The optical waveguide structure 300 includes an optical waveguide layer 306, optical confinement layers 304 and 308 sandwiching the optical waveguide layer 306, a buffer layer 302, and a capping layer 309 located on the outer sides of the optical confinement layers 304 and 308, respectively. The semiconductor laser 200 includes a first cladding layer 202, an active layer 204, a carrier barrier layer 205, a first guiding layer 206, a second guiding layer 207, and a second cladding layer 208. Light emitted from the semiconductor laser 200 is directly coupled with the optical waveguide structure 300, and propagates in the optical waveguide layer 306.
The abutting method described above eliminates the necessity of positioning the semiconductor laser and the optical waveguide with each other, thereby providing high mechanical stability, compared with a method where they are separately fabricated and then bonded together.
The above conventional method is disadvantageous in the following points.
(1) In reality, the optical waveguide structure is not formed as ideally shown in FIG. 7A in the area formed by the vertical etching, but is formed as shown in FIG. 7B, for example. That is, the optical waveguide layer 306 of the optical waveguide structure 300 is slanted from the horizontal direction in the interface area with the semiconductor laser 200. In such a slant layer area, since light is influenced by the refractive index distribution in the area, the percentage of light which is not coupled with the optical waveguide layer 306 increases. Thus, the coupling ratio is much lower than that anticipated from the ideal configuration.
(2) When the vertical beam diameter of light emitted from the semiconductor laser 200 does not match with the vertical beam diameter in a native mode of the optical waveguide structure 300, the greater the difference therebetween, the lower the percentage of light emitted from the semiconductor laser 200 which is coupled with the optical waveguide structure 300 becomes.
The above problems (1) and (2) will be described more specifically.
FIG. 7B shows the case where a GaAs/AlGaAs DFB semiconductor laser is vertically etched and then AlGaAs materials are grown in the etched area by metal organic chemical vapor deposition (MOCVD) to form the optical waveguide structure 300.
In the process of the growth of the AlGaAs materials, since the growth rate greatly depends on the plane orientation, a plane with a lower growth rate grows more slowly than a plane with a higher growth rate, resulting in the structure as shown in FIG. 7B. In this case, a slant layer structure slanted from the horizontal direction is formed at the interface between the semiconductor laser 200 and the optical waveguide structure 300. Accordingly, part of light emitted from the semiconductor laser 200 is reflected or refracted by the slant layer structure at the interface, thereby to be radiated outside the optical waveguide structure, not coupled with the optical waveguide layer 306. In other words, radiation loss occurs.
It has been confirmed from the results of experiments conducted by the inventors of the present invention that light of about 1 dB was radiated by the slant layer structure at the interface. The inventors fabricated various types of the optical waveguide structure under various different conditions. The resultant configurations of the optical waveguide structures varied depending on the conditions, but it was not possible to obtain the ideal configuration as shown in FIG. 7A. In any case, a radiation loss in the range of 0.5 to 1 dB was observed.
Moreover, in the conventional case, the thickness of the optical waveguide layer 306 of the optical waveguide structure 300 was about 2 .mu.m while the vertical beam diameter of the semiconductor laser 200 was about 1 .mu.m. This difference caused a great mode mismatch when light emitted from the semiconductor laser 200 was coupled with the optical waveguide layer 300. Due to this mode mismatch, a radiation loss of 1.7 dB was observed.
Thus, the total radiation loss amounts to about 2.7 dB. Due to this radiation loss, the semiconductor laser 200 is forced to provide a light output higher than that actually required. As a result, the power consumption of the semiconductor laser 200 increases, and moreover the reliability of the semiconductor laser 200 is reduced.